Real name: Dr. Karl Hellfern
First Appearance: Detective Comics I#29 (July 1939)
Abilities: mad scientist
Quotes: "A crime such as ours must be sure to attract his attention"
Worst Act: Murder
Created by: Bill Finger (writer) and Bob Kane (artist-credited). Gardner Fox is also sometimes credited
And here we have (arguably) the first proper super villain Batman ever faced. More of a throw back (or straight usage given the time period) to the old pulp mad scientists, Dr. Death made his debut by threatening the wealthy of Gotham City to pay him a handsome fee or else he'd poison them.
The dear doctor was also the first reappearing villain, returning from his seeming demise in Detective Comics I#30 (August 1939), only now looking like a brown skeleton thanks to his lab blowing up in the previous issue.
This would be his last appearance until 1982, when writer Gerry Conway brought him back using the same methods as before. He would stay gone until 2002, when his brought out of mothballs to face Batgirl in her own series. Since then he has mostly stayed as a background character, using popping up in scenes of mad science or various crime dens in Gotham.